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Stephen R Forrest - One of the best experts on this subject based on the ideXlab platform.
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Fast Organic Vapor Phase Deposition of Thin Films in Light-Emitting Diodes.
ACS nano, 2020Co-Authors: Kan Ding, Max Shtein, Kai Sun, Shaocong Hou, Steven Morris, Stephen R ForrestAbstract:Fast Deposition of thin films is essential for achieving low-cost, high-throughput phosphorescent organic light-emitting diode (PHOLED) production. In this work, we demonstrate rapid and uniform growth of semiconductor thin films by organic Vapor Phase Deposition (OVPD). A green PHOLED comprising an emission layer (EML) grown at 50 A/s with bis[2-(2-pyridinyl-N)phenyl-C](acetylacetonato)iridium(III) (Ir(ppy)2(acac)) doped into 4,4'-bis(N-carbazolyl)-1,1'-biphenyl (CBP) exhibits a maximum external quantum efficiency of 20 ± 1%. The morphology, charge transport properties, and radiative efficiency under optical and electrical excitation of the PHOLED EML are investigated as functions of the Deposition rate via both experimental and theoretical approaches. The EML shows no evidence for gas Phase nucleation of the organic molecules at Deposition rates as high as 50 A/s. However, the roll-off in quantum efficiency at high current progressively increases with Deposition rate due to enhanced triplet-polaron annihilation. The roll-off results from accumulation of stress within the PHOLED EML that generates a high density of defect states. The defects, in turn, act as recombination sites for triplets and hole polarons, leading to enhanced triplet-polaron annihilation at high current. We introduce a void nucleation model to describe the film morphology evolution that is observed using electron microscopy.
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continuous roll to roll fabrication of organic photovoltaic cells via interconnected high vacuum and low pressure organic Vapor Phase Deposition systems
Applied Physics Letters, 2018Co-Authors: Boning Qu, Stephen R ForrestAbstract:We demonstrate continuous roll-to-roll (R2R) fabrication of single junction and tandem organic photovoltaic (OPV) cells on flexible plastic substrates employing a system that integrates organic Deposition by high vacuum thermal eVaporation (VTE) and low pressure organic Vapor Phase Deposition (OVPD). By moving the substrate from chamber to chamber and then depositing films on stationary substrates, we achieve power conversion efficiencies of PCE = 8.6 ± 0.3% and 8.9 ± 0.2% for the single junction and tandem cells, respectively. Single junction OPVs are also fabricated on a continuously translating substrate at 0.3 cm/s, to achieve PCE = 8.5 ± 0.2%. Thin films grown on translating substrates by OVPD show <3% thickness non-uniformity and 0.66 nm root mean square surface roughness, similar to that obtained by VTE. Our results suggest that R2R film Deposition comprising multiple Vapor Deposition technologies is a promising method for rapid speed and continuous manufacturing of high quality, small molecular weight organic electronic materials.We demonstrate continuous roll-to-roll (R2R) fabrication of single junction and tandem organic photovoltaic (OPV) cells on flexible plastic substrates employing a system that integrates organic Deposition by high vacuum thermal eVaporation (VTE) and low pressure organic Vapor Phase Deposition (OVPD). By moving the substrate from chamber to chamber and then depositing films on stationary substrates, we achieve power conversion efficiencies of PCE = 8.6 ± 0.3% and 8.9 ± 0.2% for the single junction and tandem cells, respectively. Single junction OPVs are also fabricated on a continuously translating substrate at 0.3 cm/s, to achieve PCE = 8.5 ± 0.2%. Thin films grown on translating substrates by OVPD show <3% thickness non-uniformity and 0.66 nm root mean square surface roughness, similar to that obtained by VTE. Our results suggest that R2R film Deposition comprising multiple Vapor Deposition technologies is a promising method for rapid speed and continuous manufacturing of high quality, small molecular we...
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Continuous roll-to-roll fabrication of organic photovoltaic cells via interconnected high-vacuum and low-pressure organic Vapor Phase Deposition systems
Applied Physics Letters, 2018Co-Authors: Stephen R ForrestAbstract:We demonstrate continuous roll-to-roll (R2R) fabrication of single junction and tandem organic photovoltaic (OPV) cells on flexible plastic substrates employing a system that integrates organic Deposition by high vacuum thermal eVaporation (VTE) and low pressure organic Vapor Phase Deposition (OVPD). By moving the substrate from chamber to chamber and then depositing films on stationary substrates, we achieve power conversion efficiencies of PCE = 8.6 ± 0.3% and 8.9 ± 0.2% for the single junction and tandem cells, respectively. Single junction OPVs are also fabricated on a continuously translating substrate at 0.3 cm/s, to achieve PCE = 8.5 ± 0.2%. Thin films grown on translating substrates by OVPD show
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Morphology control in fullerene-based buffer layer by organic Vapor Phase Deposition
2016 IEEE 43rd Photovoltaic Specialists Conference (PVSC), 2016Co-Authors: Byeongseop Song, Stephen R ForrestAbstract:The morphology of small molecular weight organic thin film mixtures used in organic photovoltaics (OPV) is precisely controlled through growth conditions used in organic Vapor Phase Deposition (OVPD). We found that the crystallinity of C60 in 3, 5, 3, 5-tetra(m-pyrid-3-yl)phenyl[1,1]biphenyl (BP4mPy):C60 buffer layers undergoes changes as a function of chamber growth pressure. The chamber growth pressure is optimized at 0.28 torr, resulting in the largest C60 crystalline domains. The power conversion efficiency of organic Vapor Phase deposited tetraphenyldibenzoperiflanthene (DBP):C70 planar mixed heterojunction OPVs using the optimized BP4mPy:C60 buffer layer is (8.0±0.2)% compared to (6.6±0.2)% using0 amorphous buffers grown by vacuum thermal eVaporation.
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Nanoscale Control of Morphology in Fullerene-Based Electron-Conducting Buffers via Organic Vapor Phase Deposition
Nano letters, 2016Co-Authors: Byeongseop Song, Stephen R ForrestAbstract:Small molecular weight organic thin film mixtures of the electron-conducting C60 in a wide energy gap matrix, 3,5,3′,5′-tetra(m-pyrid-3-yl)phenyl[1,1′]biphenyl (BP4mPy) forms a high efficiency electron filtering buffer in organic photovoltaics (OPV). Electrons are conducted via percolating paths of C60 whereas excitons are blocked by the BP4mPy. We find that the conductivity and exciton blocking efficiency of the blends are strongly dependent on film morphology that can be precisely controlled by the conditions used in the organic Vapor Phase Deposition (OVPD). Specifically, we find that a background carrier gas pressure of 0.28 Torr leads to extended and highly conductive crystalline C60 domains. Furthermore, the structure is strongly influenced by carrier gas pressure. Via a combination of morphological measurements and molecular dynamics simulations, we find that this dependence is due to kinetically induced structural annealing at the growth interface. The highest electron mobility of (6.1 ± 0.5) × 10...
Max Shtein - One of the best experts on this subject based on the ideXlab platform.
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Fast Organic Vapor Phase Deposition of Thin Films in Light-Emitting Diodes.
ACS nano, 2020Co-Authors: Kan Ding, Max Shtein, Kai Sun, Shaocong Hou, Steven Morris, Stephen R ForrestAbstract:Fast Deposition of thin films is essential for achieving low-cost, high-throughput phosphorescent organic light-emitting diode (PHOLED) production. In this work, we demonstrate rapid and uniform growth of semiconductor thin films by organic Vapor Phase Deposition (OVPD). A green PHOLED comprising an emission layer (EML) grown at 50 A/s with bis[2-(2-pyridinyl-N)phenyl-C](acetylacetonato)iridium(III) (Ir(ppy)2(acac)) doped into 4,4'-bis(N-carbazolyl)-1,1'-biphenyl (CBP) exhibits a maximum external quantum efficiency of 20 ± 1%. The morphology, charge transport properties, and radiative efficiency under optical and electrical excitation of the PHOLED EML are investigated as functions of the Deposition rate via both experimental and theoretical approaches. The EML shows no evidence for gas Phase nucleation of the organic molecules at Deposition rates as high as 50 A/s. However, the roll-off in quantum efficiency at high current progressively increases with Deposition rate due to enhanced triplet-polaron annihilation. The roll-off results from accumulation of stress within the PHOLED EML that generates a high density of defect states. The defects, in turn, act as recombination sites for triplets and hole polarons, leading to enhanced triplet-polaron annihilation at high current. We introduce a void nucleation model to describe the film morphology evolution that is observed using electron microscopy.
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morphology control and material mixing by high temperature organic Vapor Phase Deposition and its application to thin film solar cells
Journal of Applied Physics, 2005Co-Authors: Fan Yang, Max Shtein, Stephen R ForrestAbstract:Organic Vapor-Phase Deposition is used to grow photovoltaic cells consisting of molecular compounds that require high-eVaporation temperatures. The organic crystal size and film morphology are controlled by adjusting the source eVaporation temperature, substrate temperature, carrier gas flow rate, and chamber pressure. The resulting surface morphology produces a highly folded donor–acceptor interface with a fourfold increase in area compared with a planar heterojunction, leading to an increased exciton diffusion efficiency, and hence an 80% increase in photovoltaic cell power conversion efficiency. Mixed organic layers with controlled constituent ratios and layer thicknesses are grown by the simultaneous coDeposition of multiple source materials. Photovoltaic cells comprised of a copper phthalocyanine:C60 mixed layer show comparable performance to analogous vacuum-deposited cells.
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Stable and efficient electrophosphorescent organic light-emitting devices grown by organic Vapor Phase Deposition
Applied Physics Letters, 2005Co-Authors: Theodore X. Zhou, Tan Ngo, Max Shtein, Julie J. Brown, Stephen R ForrestAbstract:An electrophosphorescent organic light-emitting device (PHOLED™) employing fac-tris(2-phenylpyridine)iridium [Ir(ppy)3] as the green emitting phosphor has been fabricated using a pilot-production organic Vapor Phase Deposition (OVPD™) system. Highly controlled mass transport of the organic Vapor to the substrate results in Deposition rates of over 10A∕s and spatial uniformity better than ±2% across a 150mm×150mm substrate with less than ±2% run-to-run variations. The device current–voltage, luminous efficiency, and operational lifetime performances are compared to those of a similar device grown by conventional vacuum thermal eVaporation (VTE). The green OVPD-grown PHOLED exhibits a maximum external quantum efficiency of (7.0±0.1)% at a luminance of 1000cd∕m2, comparable to the VTE device performance. The operational lifetime of the OVPD-grown devices was found to be comparable to or even somewhat longer than the lifetime achieved by VTE. Furthermore, PHOLEDs with emissive layers deposited at 4.8 and 10.8...
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micropatterning of small molecular weight organic semiconductor thin films using organic Vapor Phase Deposition
Journal of Applied Physics, 2003Co-Authors: Max Shtein, Peter Peumans, J Benziger, Stephen R ForrestAbstract:Using both analytical and experimental methods, we show that micron scale patterned growth of small molecular weight organic semiconductor thin films can be achieved by the recently demonstrated process of organic Vapor Phase Deposition (OVPD). In contrast to the conventional process of vacuum thermal eVaporation, the background gas pressure during OVPD is typically 0.1–10 Torr, resulting in a molecular mean free path (mfp) of from 100 to 1 μm, respectively. Monte Carlo simulations of film growth through apertures at these gas densities indicate that when the mfp is on the order of the mask-to-substrate separation, deposit edges can become diffuse. The simulations and Deposition experiments discussed here indicate that the deposited feature shape is controlled by the mfp, the aperture geometry, and the mask-to-substrate separation. Carefully selected process conditions and mask geometries can result in features as small as 1 μm. Furthermore, based on continuum and stochastic models of molecular transport in confined geometries, we propose the in situ direct patterning growth technique of organic Vapor jet printing. The high pattern definition obtained by OVPD makes this process attractive for the growth of a wide range of structures employed in modern organic electronic devices.Using both analytical and experimental methods, we show that micron scale patterned growth of small molecular weight organic semiconductor thin films can be achieved by the recently demonstrated process of organic Vapor Phase Deposition (OVPD). In contrast to the conventional process of vacuum thermal eVaporation, the background gas pressure during OVPD is typically 0.1–10 Torr, resulting in a molecular mean free path (mfp) of from 100 to 1 μm, respectively. Monte Carlo simulations of film growth through apertures at these gas densities indicate that when the mfp is on the order of the mask-to-substrate separation, deposit edges can become diffuse. The simulations and Deposition experiments discussed here indicate that the deposited feature shape is controlled by the mfp, the aperture geometry, and the mask-to-substrate separation. Carefully selected process conditions and mask geometries can result in features as small as 1 μm. Furthermore, based on continuum and stochastic models of molecular transport ...
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Micropatterning of small molecular weight organic semiconductor thin films using organic Vapor Phase Deposition
Journal of Applied Physics, 2003Co-Authors: Max Shtein, Jay Burton Benziger, Peter Peumans, Stephen R ForrestAbstract:Using both analytical and experimental methods, we show that micron scale patterned growth of small molecular weight organic semiconductor thin films can be achieved by the recently demonstrated process of organic Vapor Phase Deposition (OVPD). In contrast to the conventional process of vacuum thermal eVaporation, the background gas pressure during OVPD is typically 0.1–10 Torr, resulting in a molecular mean free path (mfp) of from 100 to 1 μm, respectively. Monte Carlo simulations of film growth through apertures at these gas densities indicate that when the mfp is on the order of the mask-to-substrate separation, deposit edges can become diffuse. The simulations and Deposition experiments discussed here indicate that the deposited feature shape is controlled by the mfp, the aperture geometry, and the mask-to-substrate separation. Carefully selected process conditions and mask geometries can result in features as small as 1 μm. Furthermore, based on continuum and stochastic models of molecular transport in confined geometries, we propose the in situ direct patterning growth technique of organic Vapor jet printing. The high pattern definition obtained by OVPD makes this process attractive for the growth of a wide range of structures employed in modern organic electronic devices.
Hua-kun Liu - One of the best experts on this subject based on the ideXlab platform.
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Uniform Polypyrrole Layer-Coated Sulfur/Graphene Aerogel via the Vapor-Phase Deposition Technique as the Cathode Material for Li-S Batteries.
ACS applied materials & interfaces, 2020Co-Authors: Mohammad Rejaul Kaiser, Yuyang Hou, Tengfei Zhou, Zhao Jun Han, Weihong Lai, Jun Chen, Zaiping Guo, Hua-kun LiuAbstract:The practical application of Li–S batteries is hampered because of their poor cycling stability caused by electrolyte-dissolved lithium polysulfides. Dual functionalities such as strong chemical adsorption stability and high conductivity are highly desired for an ideal host material for the sulfur-based cathode. Herein, a uniform polypyrrole layer-coated sulfur/graphene aerogel composite is designed and synthesized using a novel Vapor-Phase Deposition method. The polypyrrole layer simultaneously acts as a host and an adsorbent for efficient suppression of polysulfide dissolution through strong chemical interaction. The density functional theory calculations reveal that the polypyrrole could trap lithium polysulfides through stronger bonding energy. In addition, the deflation of sulfur/graphene hydrogel during the Vapor-Phase Deposition process enhances the contact of sulfur with matrices, resulting in high sulfur utilization and good rate capability. As a result, the synthesized polypyrrole-coated sulfur/...
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N-doped crumpled graphene derived from Vapor Phase Deposition of PPy on graphene aerogel as an efficient oxygen reduction reaction electrocatalyst
ACS applied materials & interfaces, 2015Co-Authors: Meng Wang, Yuyang Hou, Hua-kun Liu, Jiazhao Wang, Dongqi Shi, David Wexler, Simon D. Poynton, Robert C. T. Slade, Weimin Zhang, Jun ChenAbstract:Nitrogen-doped crumpled graphene (NCG) is successfully synthesized via Vapor Phase Deposition of polypyrrole onto graphene aerogel followed by thermal treatment. The NCG was explored as an electrocatalyst for the oxygen reduction reaction, showing comparable electrocatalytic performance with the commercial Pt/C in alkaline membrane exchange fuel cells because of the well-regulated nitrogen doping and the robust micro-3D crumpled porous nanostructure.
Cedric Rolin - One of the best experts on this subject based on the ideXlab platform.
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Laser induced fluorescence monitoring of the transport of small organic molecules in an organic Vapor Phase Deposition system
Journal of Applied Physics, 2012Co-Authors: Cedric Rolin, Garen Vartanian, Stephen R ForrestAbstract:Laser-induced fluorescence is employed for the accurate and real-time in-situ monitoring of the concentration of organic molecules in an organic Vapor Phase Deposition (OVPD) chamber. We investigate the transport dynamics of organic species in a hot N2 carrier gas from the eVaporation source to the substrate. Based on the time-dependent concentration of organic molecules obtained from their fluorescence intensity near the substrate, we find that carrier gas transport is accurately described by Poiseuille flow. The interplay between convective and diffusive forces gives rise to dispersion of organic molecules in the carrier gas, resulting in the development of plug flow as described by Taylor-Aris theory. Retention of molecules in chamber dead volumes delays transport and introduces tailing of the concentration transients. Our study indicates how OVPD system design and operating conditions can be optimized to limit the duration of transport transients, ultimately leading to precise control over the growth ...
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Growth of pentacene thin films by in-line organic Vapor Phase Deposition
Organic Electronics, 2010Co-Authors: Cedric Rolin, Karolien Vasseur, Jan Genoe, P.l. HeremansAbstract:Abstract We present the extension of the organic Vapor Phase Deposition technique to an in-line geometry, in which the sample travels underneath an elongated showerhead that sprays molecules transported by a stream of carrier gas. Highly uniform pentacene films are grown at web speeds of up to 2.1 m/min, equivalent to an average Deposition rate of 105 A/s in a static system. With transistor mobilities of up to 1.5 cm2/V s, these pentacene films are of high electrical quality. Importantly, this quality is conserved up to the highest Deposition speeds. We discuss the relationships between in-line Deposition rate, morphology and crystallinity of the deposited pentacene films and their electrical characteristics.
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High-speed growth of pentacene thin films by in-line organic Vapor Phase Deposition
Organic Field-Effect Transistors VIII, 2009Co-Authors: Cedric Rolin, Karolien Vasseur, Jan Genoe, Soeren Steudel, Peter Vicca, P.l. HeremansAbstract:Taking another step towards industrial production of devices based on organic semi-conductors, this work presents an extension of the organic Vapor Phase Deposition technique to in-line geometry. A study of the in-line tool operation is carried out. It leads to the definition of a specific in-line Deposition rate that qualifies the coating speed. It also allows for an understanding of processing parameter variations that lead to high Deposition speeds. As a consequence, pentacene films are grown at in-line Deposition rates of up to 1055μm 2 /s. This corresponds to web speeds of 2.1 m/min, equivalent to an average Deposition rate of 105 A/s in a static system. These films present a high uniformity, with a thickness standard deviation below 1.2% over 4 inch diameter substrates. Moreover, with transistor mobilities of up to 1.5 cm 2 /Vs, these pentacene films are of excellent electrical quality. This quality is conserved up to the highest Deposition rates. Finally, 5-stage ring oscillators on foil based on a pentacene thin film deposited by in-line OVPD achieve a frequency of 24 kHz at a supply voltage of 20 V.
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Functional Pentacene Thin Films Grown by In-Line Organic Vapor Phase Deposition at Web Speeds above 2 m/min
Applied Physics Express, 2009Co-Authors: Cedric Rolin, Jan Genoe, Soeren Steudel, Peter Vicca, P.l. HeremansAbstract:We show in this paper that the organic Vapor Phase Deposition technique can advantageously be extended to an in-line system, where a susceptor moves at a constant speed underneath an elongated showerhead. Highly uniform pentacene films are grown at web speeds of up to 2.1 m/min, equivalent to an average Deposition rate of 105 angstrom/s in a static system. These pentacene films are of high electrical quality as proven by transistor mobilities of up to 1.5 cm(2) V-1 s(-1) and five-stage ring oscillators on foil that achieve a frequency of 24 kHz at a supply voltage of 20 V. (C) 2009 The Japan Society of Applied Physics
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Vapor Phase Deposition of organic semiconductors for field effect transistor applications
2009Co-Authors: Cedric RolinAbstract:The coming of age of organic circuits requires the development of high-performance and cost-effective processing techniques. In this work, the Organic Vapor Phase Deposition (OVPD) of small-molecular-weight organic molecules is investigated for the production of thin semiconducting films used as the active layer of circuits. Two OVPD systems are designed and assembled: A Static OVPD system that is inspired from a chemical Vapor Phase Deposition tool and an In-line OVPD system that accommodates a linearly moving substrate and that is roll-to-roll compatible. These two tools are optimized for the growth of pentacene and PTCDI-C13, two high-mobility organic semiconductors. With saturation mobilities of up to 1.5 cm²/Vs and reproducible well-behaved characteristics, high-quality p-type transistors based on OVPD-grown pentacene films are demonstrated. High-quality circuits are also produced using these pentacene layers grown on flexible substrates. The demonstration of high-mobility n-type transistors based on OVPD-deposited PTCDI-C13 film paves the way towards the production of complementary circuits. Analytical and numerical transport models are developed in order to understand and characterize the link between processing conditions, material use efficiency, film thickness uniformity and growth speeds. A material use efficiency of up to 65% and a thickness uniformity as low as 1.1% over 4’’ substrates are demonstrated in our systems. Moreover, high production throughput is investigated by maximizing film growth speeds. A Deposition rate of 15 A/s is obtained for PTCDI-C13 in the Static system and a record average Deposition rate of 105 A/s is achieved for pentacene in the In–line system. In addition, we show that, in this system, thin film electrical characteristics are conserved up to the highest growth speeds. Finally, sub-monolayer growth of pentacene is investigated. We show that growth by OVPD follows the same patterns as vacuum growth. However, the presence of the background gas delays nucleation and enhances growth kinetics. As a conclusion, this work shows that OVPD, especially in the in-line architecture, is a very promising candidate for the low-cost production of high-performance organic semiconducting thin films.
S. K. Hark - One of the best experts on this subject based on the ideXlab platform.
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Selenium-related luminescent centers in metalorganic chemical-Vapor-Phase Deposition grown ZnSe epilayers on GaAs
Applied Physics Letters, 2001Co-Authors: X. B. Zhang, S. K. HarkAbstract:ZnSe epilayers were grown on a (001) GaAs substrate by metalorganic chemical-Vapor-Phase Deposition. An interruption of the Zn source (i.e., Se passivation) was purposely introduced during the growth. The optical properties of the epilayers grown were studied by photoluminescence (PL) spectroscopy. We show that Se passivation during the growth interruption introduces luminescent centers in the epilayers. Evidence of this assignment comes from the characteristic temperature and excitation wavelength dependence of the PL spectra, which are distinctly different from those of commonly observed deep-level emissions associated with the so-called self-activated centers. Moreover, the PL peak energy of the centers depends strongly on the coverage of Se: the longer the time or the higher the flow rate of the Se precursor used for the passivation, the lower the energy of its PL peak. The possible origin of this luminescence is discussed.
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Influence of capping layer thickness on the polarization of photoluminescence of CdSe/ZnSe quantum dots grown by metalorganic chemical Vapor Phase Deposition
Journal of Electronic Materials, 2001Co-Authors: X. B. Zhang, S. K. HarkAbstract:The polarization of the photoluminescence (PL) of self-assembled CdSe quantum dots (QDs), grown by metalorganic chemical Vapor Phase Deposition, was measured. From the (001) surface, the PL was found preferentially polarized in the \([1\bar 10]\) direction, while from the \((1\bar 10)\) cleaved surface in the [001] direction. The polarization of PL depends strongly on the ZnSe capping layer thickness and the PL energy. With an increase in ZnSe coverage, the intensity ratio \(I[1\bar 10]/I_{[110]} \) was found to increase first, then decrease after the coverage is thicker than a critical value. Moreover, such a critical thickness is smaller for larger QDs (lower PL energies). Possible origins of the PL polarization are discussed. We suggest that besides the quantum confinement effects, the strain field in the QDs also plays an essential role in the observed polarization of PL.
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Thickness dependent surface morphologies and luminescent properties of ZnSe epilayers grown on (001) GaAs by metalorganic chemical Vapor Phase Deposition
Journal of Crystal Growth, 2001Co-Authors: X. B. Zhang, S. K. HarkAbstract:Abstract ZnSe epilayers were grown on (0 0 1) GaAs substrate by using metalorganic chemical Vapor Phase Deposition. The growth was carried out by first growing a 30 nm thick ZnSe buffer layer at a lower temperature of 360°C, followed by the main growth at 450°C. Surface morphologies and photoluminescence (PL) of the ZnSe layers with various thicknesses were investigated. We found that the ZnSe buffer layer contains (1) high density of islands about 80 nm in size and (2) low density of holes about 100–1500 nm in diameter. These two features develop in different ways during the growth at higher temperature. The places containing small islands become rougher first and then smoother, with an increase in layer thickness. After the thickness exceeds 210 nm, the surface becomes atomically flat. The holes, however, favor the three-dimensional growth mode and develop into growth hillocks. Energy dispersive X-ray spectroscopy showed that the hole is mainly made up of Ga–Se compounds, consistent with the earlier reports that Ga 2 Se 3 compounds are the source for the growth of hillocks. PL study showed that the donors, acceptors and the deep level emissions associated defects are mainly concentrated at the GaAs/ZnSe interface.
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Luminescent studies of alloy ZnxCd1−xSe quantum dots grown on ZnSe by metalorganic chemical Vapor-Phase Deposition
Journal of Crystal Growth, 1999Co-Authors: X. B. Zhang, S. K. HarkAbstract:Abstract Zn x Cd 1− x Se alloy quantum dots (QDs) with x in the range 0–0.39 are grown by metalorganic chemical Vapor-Phase Deposition on ZnSe. Cathodoluminescence (CL) and photoluminescence (PL) were used to study these self-assembled quantum dots. CL imaging and spectra show that clusters of QDs are efficient luminescent sites. A large red shift of the low-temperature PL peak energy of QDs, despite an increase in Zn, is attributed to a considerable increase in their size. This increase in size is consistent with the results of recent theoretical models. In forming the self-assembled QDs, mismatch strain is regarded as the fundamental driving force. When the strain changes, through a change in the composition of Zn x Cd 1− x Se, QDs of a different size are obtained. A decrease in size, in turn, results in stronger quantum confinement effects. The size of the QDs is very sensitive to small changes of strain. Even a minute reduction in the zinc content of the QDs, achieved through a lengthening of growth interruptions, produces an observable blue shift of luminescence, as a result of the strengthening of the quantum confinement energy.
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Depth-resolved cathodoluminescence study of ZnxCd1−xSe epilayer grown on (001) InP by metal organic chemical Vapor Phase Deposition
Applied Physics Letters, 1998Co-Authors: X. B. Zhang, H. K. Won, S. K. HarkAbstract:Optical properties of zincblende structured ZnxCd1−xSe epilayer grown on InP by metal organic chemical Vapor Phase Deposition at temperatures of 360, 400, and 440 °C are investigated with low temperature cathodoluminescence spectroscopy (CL). Both near band gap and deep level emissions are found for the samples grown at 400 °C and above, but deep level emissions are absent for the sample grown at 360 °C. We conclude that the growth temperature should be kept below the temperature at which InP begins to decompose and diffusion of III–V constituents into the epilayer occurs. Evidence of this diffusion comes from an analysis of depth resolved CL studies, which shows that the deep level emissions occur mainly at the epilayer/substrate interface. By monitoring the ratio of the intensity of the deep level emissions to that of the near band emissions, we find that this ratio is larger for samples grown at high temperatures than those at low temperatures. Indium diffusion from the substrate into the epilayer is m...
